1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to a fuel cell system in which the temperature within the cell may be raised and controlled to improve performance in cold environments.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel, depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or formaldehyde, are attractive choices for fuels due to their high specific energies.
Fuel cell systems may be divided into xe2x80x9creformer basedxe2x80x9d (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) or xe2x80x9cdirect oxidationxe2x80x9d in which the fuel is fed directly into the cell without internal processing. Most currently available fuel cells are of the reformer-based type, and their fuel processing requirement limits their application to relatively large applications relative to direct oxidation systems.
An example of a direct oxidation system is the direct methanol fuel cell system or DMFC. In a DMFC, the electrochemical reaction at the anode is a conversion of methanol and water to CO2, H+ and exe2x88x92. More specifically, a carbonaceous fuel (typically methanol in an aqueous solution) is applied to a protonically-conductive, electronically non-conductive membrane in the presence of a catalyst to enable direct anodic oxidation of the carbonaceous fuel at the anode. Upon contact with the catalyst, hydrogen atoms from the fuel are separated from the other components of the fuel molecule. Upon closing of a circuit connecting the anode to the cathode through an external load the protons and electrons from the hydrogen are separated, the resulting protons pass through the membrane electrolyte, and the electrons travel through an external load. The protons and electrons combine on the cathode, with oxygen supplied to the cathode, generating water at the cathode. The carbon component of the fuel is converted into CO2 at the anode, thus generating additional protons and electrons.
Present membrane electrolytes are permeable to methanol and water. Consequently, methanol may pass through the membrane electrolyte to the cathode side without generating electricity. This phenomenon, commonly referred to as xe2x80x9cmethanol crossover,xe2x80x9d reduces the efficiency of the DFMC, and generates heat as a result of the oxidation of the xe2x80x9ccrossed-overxe2x80x9d methanol at the cathode side of the cell. Presently, methanol crossover is reduced by diluting the methanol with water, and using a methanol solution of approximately 3% methanol as fuel for a DMFC.
During optimal steady state operation, DMFCs operate at temperatures that are generally higher than ambient air temperatures, with most operating between 30xc2x0 and 80xc2x0 C., depending on the application for which the DMFC is providing power. The performance of the DMFC (and therefore the DMFC power system) is related to the temperature of the DMFC. Thus, when a DMFC has been inactive for an extended period of time or is required to operate in a cold ambient environment, the DMFC will typically not perform optimally until the cell is warmed up by heat that is generated during operation. This is particularly problematic in applications such as consumer electronic devices because such devices may be used in cold environments or are xe2x80x9coffxe2x80x9d for substantial time periods, during which time the DMFC may cool below an optimal operating temperature. It is therefore desirable to develop a system that allows a DMFC to ramp up to full operating temperature quickly to allow for generation of desired electricity as quickly as possible.
The present invention provides an apparatus and method for increasing the temperature of a fuel cell, such as a DMFC, and maintaining it at an optimal level by directing fuel to the cathode in order to cause heat-producing oxidation. By increasing the temperature in the fuel cell, xe2x80x9ccold startxe2x80x9d performance of the fuel cell is improved. In a preferred embodiment, fuel is supplied directly to the cathode side of the fuel cell through a bypass fuel assembly, rapidly generating heat within the cell. The bypass fuel assembly includes a temperature sensor, a controller and a bypass valve. The sensor detects the temperature within the cell and sends a signal indicative of the temperature to the controller. In response to the signal from the temperature sensor, the controller determines whether to direct fuel to the cathode via the bypass valve in order to raise the cell""s temperature. Alternatively, electrical or other characteristics of the fuel cell can be used to direct flow of methanol to the cathode, and eliminate the need for a temperature sensor in the fuel cell.
In accordance with a first alternative embodiment of the invention, additional fuel beyond that needed to operate the fuel cell is applied to the anode. The increased concentration of fuel in the anode accelerates crossover of fuel through the membrane electrolyte, thereby increasing the amount of fuel present in the cathode and, in turn, the amount of heat generated by oxidation.
In accordance with a second alternative embodiment of the invention, a conduit is provided between a fuel source and the anode or cathode of the DMFC. The sidewall of the conduit includes an assembly which may admit air from outside the conduit and a catalyst over which the fuel passes. As the fuel flows through the conduit, the catalyst causes oxidation of some of the fuel, thus generating heat which is carried into the anode or cathode or both to raise the temperature of the DMFC and the DMFC power system. By expanding the conduit into a series of branches, at least some of which include the catalytic assembly, and arranging a number of valves among the branches, the amount of heat generated may be more precisely controlled. Alternatively, a single branching conduit can be equipped with a metering valve that controls the flow through the conduit and thereby regulates the temperature of the DMFC.
In accordance with another aspect of the invention, the flow of fuel which is being used to raise the temperature of the DMFC is controlled with a control valve that is either electrically or thermally actuated. In the thermally actuated version, a valve may be constructed of two materials having different coefficients of expansion. When the temperature is relatively cold, the valve is open and allows fuel to flow to the DMFC, which eventually raises the temperature. As the temperature rises sufficiently high, the valve closes and cuts off the flow of fuel. As a result, the DMFC""s temperature is well regulated without the need for a temperature sensor, controller or bypass valve.